Stock buybacks in the fourth quarter fell 23% on a quarter-over-quarter basis to $91.5 billion, according to preliminary data from S&P Indices. This marked the first down period since the second quarter of 2009.

Companies for years have been using buybacks to reduce their shares outstanding, which can help boost earnings per share and ultimately propel share prices higher. Such efforts had increased in popularity in the first nine months of 2011 as companies deployed huge cash stockpiles to satisfy investor demands for return.

But a top-heavy stock market prompted companies to reign in buyback plans. The thinking may be to wait for stock prices to come off recent highs before buying back shares again.

“Companies finally got it right,” said Howard Silverblatt, senior index analyst at S&P Indices. “Bought more in the bear Q3 and less in the bull Q4.”

Signaling either Peak Oil or Peak Apple, Apple earlier today topped Exxon Mobil as the largest US company by market cap.

Both mammoth companies are worth about $341 billion right now, and Exxon Mobil is at the moment slightly larger. For now.

Apple has gained a lot of ground on the oil giant since late July, when its stock price spiked. It has fallen during the market meltdown, but Exxon’s has fallen harder, and didn’t climb as much ahead of time.

Even today, Exxon is down a bit, while Apple is up 4%.

Each company has a market cap larger than the GDP of Thailand. If they teamed up — iOil? — they’d be bigger than Switzerland.

Update: Exxon Mobil ended the day with the crown, at $348 billion in market cap. Apple ended at $346.7 billion. To be continued.

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